1. Field of the Invention
The present invention relates to a plunger pump of a quick pressure-rise type utilized for increasing the pressure of working fluid quickly when the working fluid is supplied under pressure to a fluid actuator such as a hydraulic cylinder, which is adapted to be actuated by means of fluid pressure.
2. Background of the Prior Art
Generally, in order to supply the working fluid under pressure to fluid actuator, only one plunger pump is driven operatively by means of a prime mover such as an electric motor, a pneumatic piston engine, the manual power and the like as taught, for example, in to U.S. Pat. No. 4,202,514.
However, there is a disadvantage associated with such prior art, art devices, i.e., it takes a long time to increase the pressure of working fluid to a predetermined value because the load for the plunger pummp increases progressively as the pressure of working fluid rises thereto.
As for a plunger pump which aims to eliminate such a disadvantage, the one shown in FIG. 5 is of a known type. This known prior plunger pump, by the inventor of the present invention, has the following basic construction. A high pressure plunger 52a for a high pressure plunger pump 52 and a quick-charging plunger 53a for a quick-charging plunger pump 53 are connected interlockingly to one prime mover 54 so as to be driven synchronously. There there is provided an unloader valve 79 in a quick-charging interruption drain passage 78 communicated to a quick-charging plunger chamber 57. A pilot passage 80 for operating the unloader valve 79 is connected to a high pressure delivery passage 74 of a high pressure chamber 58. Until the pressure in the high pressure delivery passage 74 is increased to a predetermined pressure for a quick-charging interruption after the the plungers 52a, 53a are driven synchronously by the prime mover 54, the unloader valve 79 is kept closed and a large amount of working fluid is adapted to be supplied under a low pressure from both the quick-charging plunger chamber 57 and the high pressure plunger chamber 58 to an actuator chamber 96. When the pressure in the high pressure delivery passage 74 is increased to the predetermined pressure, the unloader valve 79 is opened and the quick-charging plunger chamber 57 is unloaded so that a small amount of working fluid is supplied under a high pressure only from the high pressure plunger chamber 58 thereto.
The plunger pump having such a basic construction functions, for example as shown in FIG. 4, while the pressure in the actuator chamber 96 is increased from the cramping primary stage pressure P.sub.o to the cramping predetermined pressure P.sub.2 under the condition that the extension of a cramping oil hydraulic cylinder 95 toward its cramp side is completed.
At the low pressure range A, from the primary pressure P.sub.o to the quick-charging interruption predetermined pressure P.sub.1, the working fluid in the actuator chamber 96 is pre-pressurized quickly because a large amount of the working fluid is delivered from both the plungers 52a, 53a. When the delivery pressure reaches a predetermined pressure P.sub.1 and the load is increased, the unloader valve 79 functions so that only the high pressure plunger pump 52 delivers the working fluid under a high pressure to the oil hydraulic cylinder 95. As the result, the delivery amount of the working fluid from the plunger pump 51 becomes less, and the delivery pressure is increased powerfully under a reduced load at the high pressure range B from the quick-charging interruption predetermined pressure P.sub.1 to the cramping predetermined pressure P.sub.2.
Accordingly, the pressure rising time is shortened by the time which is attained for shortening to the quick pre-pressurization at the low pressure range A.
As shown in FIG. 5, the above-mentioned basic plunger pump has a quick-charging pressure 73 connected to the high pressure delivery passage 74. When the unloader valve 79 operates, the quick-charging interruption drain passage 78 is merely adapted to connect the quick-charging plunger chamber 57 to a working fluid reservoir 98.
There, there are a number of disadvantages associated with the above-mentioned prior art.
(1) An air removing means is required for removing air from the high pressure plunger chamber 58, because the air mixed with the working fluid as well as intruded through a sealing part such as a seal packing 60 is apt to remain within the high pressure plunger chamber 58 during the non-operation of the plunger pump 51 and to adversely affect the high pressure plunger pump 52, for example so that it takes a long time to increase the delivery pressure or becomes impossible to do that.
When the air removing means is required in order to solve such a problem, the construction of the plunger pump 51 gets complicated by such an added means and, also, the operation thereof gets laborious.
(2) An air removing means is required for removing air from the quick-charging plunger chamber 57.
Since the unloader valve 79 is kept opened during the operation for a small amount of fluid delivery as a high pressure delivery, the quick-charging plunger chamber 57 is supplied with the working fluid also through the quick-charging interruption drain passage 78. As the result, it becomes difficult for the working fluid within the quick-charging plunger chamber 57 to be displaced, and the remaining amount of air increases gradually.
Accordingly, the air removing means is required for prevent the pressure rise from being obstructed by the remaining air at each successive starting. However, the air removing means causes the same troubles as ones mentioned in the item of (1).
(3) Foreign bodies are apt to be sucked into the quick-charging plunger chamber 57.
During the operation of a small amount of flow and high pressure delivery, the working fluid within the reservoir 98 is sucked into the quick-charging plunger chamber 57 through the quick-charging interruption drain passage 78 together with any foreign bodies that may be mixed therewith. As a result, the foreign bodies tend get between the slide surfaces of the quick-charging plunger 53a and the high pressure plunger 52a and then to scratch the slide surfaces, which may cause pressure leakages thereon as well as increase sliding resistance.